1. Field of the Invention
The present invention relates to a keyless drill chuck.
2. Description of the Prior Art
A common keyless drill chuck holds a cutting tool, such as a drill or a tap by manually fastening the chuck. The ability of the chuck to grip the cutting tool is determined by a frictional force exerted between the jaws of the chuck and the cutting tool. If the cutting tool slips between the jaws, the chuck needs to be fastened more tightly, or the machining resistance is reduced by reducing the quantity of the cut.
Japanese Utility Model Application Laid-open No. Sho-50-99585 (Prior Art 1) discloses a mechanism to prevent the free-turning of a drill in the drill chuck. Inside this known mechanism, a wedge-shaped recess is formed in the front end surface of the chuck arbor, and the end of a drill shank is formed so as to match the wedged-shaped recess. The insufficient force of the chuck to grip the drill is compensated by fitting the shank of the drill into the wedge-shaped recess in a butt-joined manner so as to prevent the free turning of the drill.
Japanese Utility Model Application Laid-open No. Hei-3-29056 (Prior Art. 2) discloses a drill gripping structure. The drill gripping structure is comprised of a main body that is removable concentrically with reference to a rotary spindle; a jaw-interval control section attached to the front portion of the main body; and a plurality of jaws which are housed in the jaw-interval control section and are positioned adjacent to each other in the circumferential direction of the jaw-interval control section. The outer circumferential surface of the drill shank is gripped by means of jaws in such a way that the axis of the drill is brought in to alignment with the axis of the main body. The drill shank has a circular cross section and has a protuberance formed at the distal end. The protuberance is comprised of a first semicylindrical section and a first rotation transmission plane in alignment with the axis of the shank. An opening is axially fluted in the front end of the main body, and the semicylindrical protuberance is fitted into the fluted opening. This opening has a second rotation transmission plane which is aligned with the rotational axis of the main body and with the first rotational axis of the semicylindrical protuberance and comes into surface-contact with the first rotation transmission plane. Further, the opening has a second semicylindrical section that is formed so as to be opposite to the first semicylindrical protuberance of the shank. In short, a semicylindrical recess is formed in the chuck.
If a cutting tool, such as a drill or a tap, slips between the jaws of the keyless drill chuck for chucking the tool, it may cause: abrasion of the jaws, damage to the jaws, run-out, machining failures, or breakage of a machine, jig or cutting tool. A cutting tool, such as a drill, whose shank is damaged, is apt to cause run-out, and hence it cannot be used in high-precision machining operations.
If the jaws are slightly abraded as a result of slippage between the cutting tool and the jaws, the keyless drill chuck remains usable, but cannot be used for precision machining because of the loss of initial precision of the drill chuck, or a reduction in the gripping force of the jaws will result in a failure in unmanned machines.
For these reasons, the chuck of the keyless drill chuck is fastened more tightly, or the quantity of cut is reduced to thereby decrease the machining resistance of a cutting tool such as a drill. There is a limit to an increase in the gripping force of the chuck. If the chuck is tightened excessively, the chuck will break. In contrast, if the quantity of cut is reduced, it takes longer, which in turn adds to the machining cost. Further, this method cannot be applied to a cutting tool used in heavy-weight machining operations.
With a keyless drill chuck, the gripping force of the jaws depends on the machining resistance. Therefore, if the chuck is manually fastened with a small force at the outset, or if the jaws are abraded, the cutting tool causes slippage, thereby resulting in an insufficient force to grip the cutting tool.
To prevent the foregoing problems, several methods have been contrived; namely, the method described in prior art 1 in which a wedge-shaped recess is formed in the front end surface of the chuck arbor; or the method described in prior art 2 in which a semicylindrical portion is formed in the chuck in order to prevent the slippage of the cutting tool.
In prior art 1, when an attempt is made to grip the shank of a cutting tool with the jaws of the chuck, the cutting tool is pushed toward its front end as a result of forward movement of the jaws of the chuck, associated with the fastening action of the cutting tool, because of the wedge-shaped recess formed in the front end surface of the chuck arbor. However, the chuck arbor does not move forward in conjunction with the forward movement of the chuck arbor, so the rear portion of the cutting tool is likely to depart from the front end surface of the chuck arbor. The smaller the diameter of a cutting tool to be retained, the further forward the jaws of the chuck will move. As a result, the rear portion of the cutting tool gradually becomes difficult to come into contact with the chuck arbor. If the cutting tool experiences physical shock, the shock directly acts on the recess. Since the outer circumference of the recess is not supported at all, the recess becomes fragile.
Prior art 1 is intended to be applied to a common drill chuck but not to a keyless drill chuck in which a through core hole is not formed. In terms of ease of machining, directly forming in the chuck arbor a recess which needs a hardness of HRC 50 or more, results in deterioration of productivity.
In prior art 2, since the gravity center of the chuck, which causes the rear portion of a cutting tool to fit into the semicylindrical recess is offset from the center of the chuck, the cutting tool eccentrically rotates at a high speed. The rear end portion of the cutting tool to be fitted into the semicylindrical recess is machined at right angles. However, a grinder usually has a rounded tip end, and hence it is difficult to grind the semicylindrical recess to its deepest portion at right angles. In order to ensure sufficient contact between the recess and the rear portion of the cutting tool, it is necessary to machine the recess sufficiently deeper than the radius of the rounded portion of the grinder. As a result, there is left only a small contact area, and hence the method disclosed in prior art 2 cannot be applied to a small-diameter drill or a fragile tap. Further, a tap handle cannot be used with a tap shank which has been machined by half.
When the rear portion of the cutting tool is fitted into the semicylindrical recess, the cutting tool is supported by the flat surface formed on one side of the rear portion. The semicylindrical reverse side of the rear portion is not supported, and hence force unevenly acts on the cutting tool, as a result of which the cutting tool is apt to be twisted. In the case of taps, they will be broken. In a case where the rear portion of the cutting tool is set to the semicylindrical recess, if a center drill is inserted into the recess while the jaws of the chuck are widely opened, or if a drill having a radius smaller than the radius of the semicylindrical recess is inserted between the jaws, the overall rear portion of the cutting tool is deeply fitted into the recess. As a result, the cutting tool is retained while the center of the chuck is kept out of alignment with the center of the cutting tool, thereby resulting in run-out or dislodgment of the cutting tool.